The invention relates to an electrochemical stripping method. In particular, the invention relates to electrochemical stripping of metallic coatings, including aluminides, from surfaces of metallic and non-metallic components.
Stripping of metallic coatings is an important step in a number of manufacturing process, including turbine component repair. Metallic coatings are provided on articles, such as turbine components, to provide protection, for example environmental protection, to the article. Removal of a metallic coating permits at least one new coating to be applied to an article, such as a turbine component, to restore its protective properties for subsequent use. Metallic coatings are typically formed with a thickness in a range from about 5 micrometers to about 500 micrometers. The composition of diffusion or overlay metallic coatings on turbine components typically includes, but is not limited to, at least one of nickel aluminide (NiAl), platinum aluminide, MCrAlY, where M can be a combination of cobalt (Co), nickel (Ni), and iron (Fe), and modifications thereof.
A stripping process should be sufficiently selective, meaning that the stripping process removes only intended materials, while preserving an article""s desired structures. For example, stripping processes should remove metallic coatings from the turbine component without consuming or otherwise modifying the underlying base alloy (also referred to as a xe2x80x9cparent alloyxe2x80x9d). Thus, the turbine component""s structural integrity will be maintained after the stripping process. In addition, internally coated surfaces and cooling holes in a turbine component must often be preserved during a stripping process.
Chemical etching is one known method for stripping metallic coatings, such as aluminide coatings, from turbine components. In chemical etching processes, a turbine component is submerged in an aqueous chemical etchant. All exposed metallic surfaces of the turbine component are in contact with the chemical etchant. The metallic coatings on the turbine component""s surfaces are corroded away by the chemical etchant by reactions known in the art. While a chemical etching process is sometimes adequate for removing metallic coatings from turbine components, some chemical stripping processes are xe2x80x9cnon-selective,xe2x80x9d meaning that the striping process does not differentiate between the coating and the underlying parent alloy. Chemical etching can thus lead to undesirable material loss, including changes in critical dimensions, such as wall thickness and cooling hole diameter, and/or structural degradation of the parent alloy, such as intergranular attack, (hereinafter, these phenomena will be referred to as xe2x80x9cmodification of the parent alloyxe2x80x9d). Chemical etching can also lead to stripping of internal passages and cooling holes of the turbine component (often referred to as xe2x80x9cinternalsxe2x80x9d). Thus, a deficiency of conventional chemical etchants is that they can not be readily directed to appropriate locations and cannot sufficiently distinguish between the coating and parent alloy, leading to degradation of the turbine component performance and reliability. In a worst-case scenario, a turbine component may be rendered unusable and scrapped.
Known chemical stripping processes may include masking of certain turbine component structures, for example internal cooling passages or holes, to overcome the non-selective nature of the processes. Masking protects against widening of cooling holes and internal passages of an article being stripped, and also prevents removal of internal coatings, however, the masking and removal thereof are time and labor consuming, imposing unwanted cost and time to a chemical stripping process.
Further, some chemical stripping processes may also operate at elevated temperature and/or pressure, and some, if not most, use hazardous chemicals, which require expensive treatment and/or disposal. These features of chemical stripping processes add additional operating costs, equipment, and safety risks, all of which are undesirable.
Electrochemical stripping processes have been disclosed for removing coatings, however, these processes are non-selective. These processes also rely on highly acidic electrolytes and any current, which is applied, can accelerate the stripping by the acid that inherently occurs. These acidic electrolyte stripping processes can result in significant damage to the parent alloy.
Therefore, it is desirable to provide a stripping process that avoids the above-noted deficiencies of known chemical stripping processes. It is desirable to provide a stripping process for an article that is chemically selective; minimizes or completely eliminates the need for masking; does not employ hazardous chemicals; and preserves the structural and dimensional integrity of the parent alloy, internal passages, and cooling holes. Further, it is desirable to provide a stripping process that exhibits a shortened process cycle time with an associated reduction in costs.
The invention sets forth an electrochemical process that strips at least one coating from an article, in which the article is formed of a parent alloy having a composition that is distinct from the coating. The coating is stripped from the parent alloy by the electrochemical process, leaving the parent alloy essentially unaffected. The electrochemical process comprises providing an electrolyte; disposing the coated article and at least one electrode in the electrolyte; applying a current from a power source between the at least one electrode and the coated article; and removing the at least one coating from the article without modifying the parent alloy.
The invention also provides a system for an electrochemical stripping process. The system comprises an electrolyte; a direct current source; and at least one electrode in which a current flow may be established. The system provides for the removal of the at least one coating from the article by causing electrochemical reactions between the coating and electrolyte upon passage of the current. Furthermore, the removal of the at least one coating occurs with minimal modification of the parent alloy.
Another aspect of the invention sets forth an electrochemical stripping process for stripping at least one coating from a coated article, in which the article comprising a parent alloy and the at least one coating is stripped from the parent alloy by the electrochemical stripping process and the parent alloy remains essentially un-modified after the electrochemical stripping process. The electrochemical stripping process comprises providing an electrolyte that comprises a charge-carrying component and a solvent, the charge-carrying component comprises sodium chloride and the solvent comprises water; disposing the coated article and at least one electrode in the electrolyte; applying a current from a power source between the at least one electrode and the coated article; and removing the at least one coating from the coated article without modifying the parent alloy.
A further aspect of the invention sets forth an electrochemical stripping process for stripping at least one coating from a coated article, in which the article comprising a parent alloy and the at least one coating is stripped from the parent alloy by the electrochemical stripping process and the parent alloy remains essentially un-modified after the electrochemical stripping process. The electrochemical process comprises providing an electrolyte that comprises a charge-carrying component and a solvent, the charge-carrying component comprises a mixture of sodium carbonate and sodium bicarbonate and the solvent comprises water; disposing the coated article and at least one electrode in the electrolyte; applying a current from a power source between the at least one electrode and the coated article; and removing the at least one coating from the coated article without modifying the parent alloy.
A further aspect of the invention sets forth an electrochemical stripping process for stripping at least one coating from a coated article, in which the article comprising a parent alloy and the at least one coating is stripped from the parent alloy by the electrochemical stripping process and the parent alloy remains essentially un-modified after the electrochemical stripping process. The electrochemical process comprises providing an electrolyte that comprises a charge-carrying component and a solvent, the charge-carrying component comprises sodium chloride and the solvent comprises propylene glycol; disposing the coated article and at least one electrode in the electrolyte; applying a current from a power source between the at least one electrode and the coated article; and removing the at least one coating from the coated article without modifying the parent alloy.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.